The invention relates to optical waveguide devices and to methods of fabricating optical waveguide devices.
In the realisation of passive integrated optical devices there is often a preferred waveguide structure for each of the building blocks of the device. For example, it is easier to fabricate a high quality directional coupler in a low refractive index contrast waveguide. Refractive index contrast, or index contrast for short, refers to the size of the refractive index difference between the core material and cladding material of the waveguide. A low index contrast is also preferable for fabricating many other optical component building blocks. By contrast, a bent or curved waveguide will tend to require a high refractive index contrast waveguide. Generally, the smaller the bend radius, the larger the refractive index contrast that will be required to keep bend losses to an acceptable level.
Some optical devices include a mixture of components, some of which that are best made with low index contrast waveguides and others of which that are best made with high index contrast waveguides. If such a device is to be made integrated on a single chip or substrate, there is then the problem of how to satisfy the conflicting requirements of the different components of the device.
One example of a device with such conflicting requirements is a ring resonator. Ring resonators are described in references [1],[2],[3] and [4]. These known ring resonators comprise a ring waveguide in combination with one or two straight waveguides that are arranged tangentially to the ring waveguide in order to couple to a circumferential portion of the ring waveguide. In this case, the coupling region is increasingly difficult to fabricate as the index contrast increases, whereas without sufficient index contrast bending losses in the ring will be too high.
Prior art micro-ring resonator designs are now reviewed in more detail. Large radius ring resonators, with radius values of the order of millimetres, have been realisable for some time. Large radius ring resonators are useful for the characterisation of the propagation losses of low index contrast waveguides and for the extraction of a specific emission line from a laser. These devices have been realised in planar lightwave circuit (PLC) technology, with low propagation loss and good fibre coupling. However, because of the low refractive index contrast of such devices, the minimum bend radius achievable is quite large, being several millimetres, and the free spectral range (FSR) is consequently quite small, for example only a few GHz using conventional glass on silicon technology with an index contrast of 0.7% (e.g. 8–12.5 GHz in reference [10]).
To be able to use a ring resonator device as a filter in a wavelength division multiplexed (WDM) system for signal processing, a much larger FSR would be needed, which in turn would require a much smaller bend radius of the ring resonator. A bend radius of a few hundreds of micrometers or less is necessary.
Small radius micro-cavity ring and disk resonators with a radius of 5 micrometers have been demonstrated with deep etched GaAs/AlGaAs waveguides [2]. However, such waveguides are less than half a micrometer wide and present very high propagation and fibre-coupling losses. They also require an excellent nanofabrication process, based on electron-beam lithography and inductive-coupled plasma ion etching in order to achieve the necessary very-deep, ultra-smooth and vertical sidewalls with gaps as small as 150 nm and as deep as 2.5 μm. The realisation of a good quality directional coupler is a critical issue in high index contrast waveguides, such as these deep etched GaAs/AlGaAs waveguides.
Micro-ring glass resonators have also been demonstrated in the form of an air-clad glass micro-ring vertically coupled to buried channel waveguides [3]. An integrated optic ring resonator has also been fabricated with two stacked layers of silica waveguides on silicon [4]. In both references [3] and [4], buried waveguide channels serve as input/output waveguides, allowing both low fibre-coupling losses and a controlled mode-coupling in the directional-coupler region. However, these methods require two steps of both deposition and etch of the waveguide core, and the alignment of the second waveguide core to the first one is very critical.
Apart from ring resonators, there are many other devices that need low index contrast, but would benefit from the possibility of having low-loss, tightly bending waveguide sections. As an example, it is well known that for filter design, the higher the order, the better shaped is the transfer function [5]. But the higher the order of the filter the longer the device. However, there is always a limited wafer size in an integrated optic process, which may limit the number of stages of the filter. As a concrete example, a 1-from-4 add/drop filter realised by a cascade of Mach-Zehnder interferometers with ⅚ stages (and SiON technology) requires a chip with dimensions of 75×5.6 mm [6]. This clearly requires a large wafer size. The design constraint of wafer size could be removed if the (linear) device could be divided into sub-sections, each sub-section being connected by a low-loss bent waveguide. But this is not generally possible, since a device component such as a Mach-Zehnder interferometer needs a low index contrast fabrication for good quality, which in turn precludes being able to provide low-loss bent waveguides with a small enough bend radius.
Still more generally, for any integration process, it is desirable to pack as many devices onto a single chip. In this respect, integration of planar optical devices is no different from other forms of integration [7]. For waveguide devices, high packing densities will be difficult to achieve without low-loss tight waveguide bends.
It would therefore be highly desirable in the fabrication of ring resonators, optical add/drop multiplexers and a variety of other optical devices, if high and low refractive index contrast waveguide sections could be conveniently combined on a single chip. The high refractive index contrast waveguide sections could then be used for low-loss tight bends, whereas the other device components could be realised in low refractive index contrast waveguide sections to optimise their quality.